Fuel channel isotope irradiation at full operating power

ABSTRACT

A method of a method of irradiating a target material in a heavy water reactor for the production of an isotope, including the steps of providing a target comprised of a material suitable for producing the isotope by way of a neutron capture event, placing the target in a primary fluid side of the heavy water reactor, and irradiating the target.

CLAIM OF PRIORITY

This application claims priority to U.S. Provisional Application No.62/540,444, filed Aug. 2, 2017, the entire disclosure of which isincorporated by reference herein.

TECHNICAL FIELD

The presently-disclosed invention relates generally to systems forirradiating radioisotope targets in nuclear reactors and, morespecifically, to systems for irradiating radioisotope targets in heavywater-moderated fission-type nuclear reactors.

BACKGROUND

Technetium-99m (Tc-99m) is the most commonly used radioisotope innuclear medicine (e.g., medical diagnostic imaging). Tc-99m (m ismetastable) is typically injected into a patient and, when used withcertain equipment, is used to image the patient's internal organs.However, Tc-99m has a half-life of only six (6) hours. As such, readilyavailable sources of Tc-99m are of particular interest and/or need in atleast the nuclear medicine field.

Given the short half-life of Tc-99m, Tc-99m is typically obtained at thelocation and/or time of need (e.g., at a pharmacy, hospital, etc.) via aMo-99/Tc-99m generator. Mo-99/Tc-99m generators are devices used toextract the metastable isotope of technetium (i.e., Tc-99m) from asource of decaying molybdenum-99 (Mo-99) by passing saline through theMo-99 material. Mo-99 is unstable and decays with a 66-hour half-life toTc-99m. Mo-99 is typically produced in a high-flux nuclear reactor fromthe irradiation of highly-enriched uranium targets (93% Uranium-235) andshipped to Mo-99/Tc-99m generator manufacturing sites after subsequentprocessing steps to reduce the Mo-99 to a usable form, such astitanium-molybdate-99 (Ti—Mo99). Mo-99/Tc-99m generators are thendistributed from these centralized locations to hospitals and pharmaciesthroughout the country. Since Mo-99 has a short half-life and the numberof existing production sites are limited, it is desirable both tominimize the amount of time needed to reduce the irradiated Mo-99material to a useable form and to increase the number of sites at whichthe irradiation process can occur.

There at least remains a need, therefore, for a system and a process forproducing a titanium-molybdate-99 material suitable for use in Tc-99mgenerators in a timely manner.

SUMMARY OF THE INVENTION

One embodiment of the present disclosure provides a method ofirradiating a target material in a heavy water reactor for theproduction of an isotope, including the steps of providing a targetcomprised of a material suitable for producing the isotope by way of aneutron capture event, placing the target in a primary fluid side of theheavy water reactor, and irradiating the target.

Another embodiment of the present disclosure provides a fuel bundlesurrogate for the irradiation of a target material in a heavy waterreactor for the production of an isotope, the fuel bundle surrogateincluding a plurality of tube sheaths, each tube sheath being parallelto a longitudinal center axis of the fuel bundle surrogate, a pluralityof end caps, each end cap being disposed on a corresponding end of atube sheath, a pair of end plates, wherein the end plates are disposedat opposing ends of the plurality of tube sheaths, and a first targetcomprised of a first target material suitable for producing the isotopeby way of a neutron capture event, wherein the first target is disposedin a first tube sheath of the surrogate fuel bundle.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate one or more embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not all,embodiments of the invention are shown. Indeed, this invention may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements.

FIGS. 1A through 1C are various views of a heavy water moderated fissionreactor and corresponding vessel penetrations;

FIGS. 2A and 2B are a perspective view and an end view, respectively, ofa fuel bundle surrogate in accordance with an embodiments of the presentinvention;

FIGS. 3A and 3B are a perspective view and a cross-sectional view,respectively, of a radioisotope target in accordance with an embodimentof the present invention;

FIGS. 4A through 4J are schematic views of a radioisotope targetirradiation process in accordance with an embodiments of the presentinvention;

FIGS. 5A through 5I are schematic views of a radioisotope irradiationprocess in accordance with an alternate embodiment of the presentinvention;

FIGS. 6A through 6G are perspective views of a transfer system formoving irradiated fuel bundle surrogates within a shielded flask, inaccordance with an embodiment of the present invention;

FIGS. 7A and 7B are cross-sectional views of a prior art tube sheath andassociated fuel pellets as found in known fissile fuel bundles;

FIGS. 8A and 8B are partial cross-sectional views of tube sheaths andtheir corresponding end caps, in accordance with an embodiment of thepresent invention;

FIGS. 9A and 9B are partial cross-sectional views of the tube sheathembodiment shown in FIG. 8A;

FIG. 10 is a partial view of a fuel bundle surrogate in accordance withthe present invention, showing various means of identifying the contentsof the fuel bundle surrogate;

FIG. 11 is a partial cross-sectional view of a prior art tube sheath fora known fissile fuel bundle;

FIG. 12 is a partial cross-sectional view of a tube sheath andassociated irradiation targets in accordance with an embodiment of thepresent invention;

FIGS. 13A through 13C are side views of various embodiments of modularfuel bundle surrogates in accordance with embodiments of the presentinvention;

FIGS. 14A through 14D are partial perspective views of a tube sheath andcorresponding end cap of a modular fuel bundle surrogate in accordancewith an embodiment of the present invention;

FIGS. 15A through 15C are exploded, partial perspective views of tubesheaths and their corresponding end caps for use with modular fuelbundle surrogates in accordance with embodiments of the presentinvention; and

FIGS. 16A through 16C are exploded, partial perspective views of tubesheaths and their corresponding end caps for use with modular fuelbundle surrogates in accordance with embodiments of the presentinvention

Repeat use of reference characters in the present specification anddrawings is intended to represent same or analogous features or elementsof the invention according to the disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to presently preferred embodiments of theinvention, one or more examples of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation,not limitation, of the invention. In fact, it will be apparent to thoseskilled in the art that modifications and variations can be made in thepresent invention without departing from the scope and spirit thereof.For instance, features illustrated or described as part of oneembodiment may be used on another embodiment to yield a still furtherembodiment. Thus, it is intended that the present invention covers suchmodifications and variations as come within the scope of the appendedclaims and their equivalents.

Referring now to the figures, the present disclosure relates to devicesand methods for irradiating radioisotope targets for the production ofradioisotopes as used in nuclear medicine. More specifically, referringnow to the figures, the irradiation of the radioisotope targets 122(FIGS. 3A and 3B) preferably occurs within a fuel channel 102 of acorresponding heavy water moderated nuclear fission reactor 100 (such asa CANDU (CANada Deuterium Uranium) reactor shown in FIGS. 1A through1C). Preferably, the materials to be irradiated, such as Mo-99, arecontained in a fuel bundle surrogate 110, as shown in FIGS. 2A and 2B.Of note, the present method of irradiating materials is of particularinterest for those materials that have extended irradiation periods,such as Lutetium-177. For the present embodiment, the fuel bundlesurrogate 110 differs primarily from a traditional fuel bundle thatincludes fissile material in that the fissile material is replaced bythe radioisotope material that is to be irradiated. As such, the fuelbundle surrogate 110 includes a plurality of tube sheaths 112 thatextend between a pair of opposing end plates 120. The end of each tubesheath 112 is enclosed by an end cap 114 which is welded to acorresponding end plate 120. The fuel bundle surrogate 110 isconstructed using the same materials, such as zirconium, andmethodologies that are currently used to construct the fissile fuelbundles 121. As these methods and materials are well known, they are notaddressed in greater detail here in the interest of brevity.

Referring now to FIGS. 3A and 3B, the radioisotope target material thatis inserted into the corresponding tube sheaths 112 of the fuel bundlesurrogate 110 is provided in the form of individual targets 122. Eachtarget includes an enclosed outer capsule 124 in which target material126 is disposed. Preferably, the outer capsule 124 is also constructedof the target material and the outer diameter of outer capsule 124allows targets to be readily removed from the tubes sheaths 112 afterirradiation by sliding action. Note, in alternate embodiments, thetarget material may be introduced into the tube sheaths 112 without useof an outer capsule, i.e., it is introduced directly into the interiorof the tube sheaths. Moreover, the radioactive target material, which ispreferably Mo-99, can be in the form of a powder, wafers, annular disks,pellet form, etc.

Each fuel bundle surrogate 110 is inserted into a pressure tube 104(FIG. 4A) of a corresponding fuel channel 102 on the primary fluid sideof the reactor 100 with an existing fueling machine 106 of the reactor100. As shown in FIG. 1C, the fueling machine 106 includes a chargemachine 109 and an accept machine 108, each of which is configured tointeract with a corresponding set of fuel channel end fittings 103 a and103 b, respectively, that are disposed on opposing ends of the pluralityof pressure tubes 104. As shown in FIG. 1C, charge machine 109 isdisposed on the “upstream” side of the reactor core 101 (meaning primarycoolant flows through reactor core 101 from left to right as shown inFIG. 1C) and accesses each pressure tube 104 by way of a correspondingfuel channel end fitting 103 a, whereas accept machine 108 is disposedon the “downstream” side of reactor core 101 and accesses the desiredpressure tube 104 through the corresponding fuel channel end fitting 103b. Each fuel bundle surrogate 110 is inserted into, and arranged within,the corresponding fuel channel 102 along with other fuel bundlesurrogates 110, fissile fuel bundles, and non-fissile placeholders 133(FIG. 4A) by the existing fueling machine 106 of the reactor 100.

After an appropriate residency time in the flux field of the reactorcore 101 for the material being irradiated, each fuel bundle surrogate110 is removed from its corresponding fuel channel 102 utilizing acceptmachine 108 and charge machine 109, as discussed in greater detailbelow. The post-irradiation fuel bundle surrogates 110 are carried to anapproved exit location, existing fuel handling port (e.g., spent fuel,ancillary, maintenance) or purpose built device, such as a trolleymounted shielded flask 130 (shown in FIGS. 6A through 6G). The shieldedflask 130 allows for multiple paths to be used when transferring theirradiated fuel bundle surrogates 110 from the fuel handling equipment(i.e. fueling machine 106) to laboratories and/or processing facilitiesthat are on-site of the reactor complex. Utilization of existing fuelhandling systems (e.g., fueling machine 106, trolleys, conveyors, newand irradiated fuel ports, etc.) allows for expedited on-line harvestingof the irradiated fuel bundle surrogates 110 during full poweroperations of the reactor 100. Moreover, in that preferred fuel bundlesurrogates 110 are constructed similarly to present fissile fuel bundlesand are handled utilizing existing fuel handling equipment, notablyaccept machine 108 and charge machine 109, methodologies of manipulatingthe fuel bundle surrogates 110 are time-tested.

Referring now to FIGS. 4A through 4J, in accordance with one examplemethod for irradiating radioisotope targets disposed in a fuel bundlesurrogate 110 a, accept machine 108 of the reactor's fuel handlingequipment is loaded with a first “half-string” of non-fissile bundles133 a, as shown in FIG. 4A. As shown in FIG. 4B, the first half-stringof non-fissile bundles 133 a is inserted into the pressure tube 104 of adedicated fuel channel 102 of the reactor core 101 in a “fuel againstflow” manner. The term “fuel against flow” means the bundles are urgedinto the pressure tube 104 of the reactor core 101 in the oppositedirection of the flow of the primary coolant, the primary coolant in thepresent example flowing from left to right as indicated by arrows 107.The accept machine 108 and charge machine 109 access the desired fuelchannel 102 by way of the corresponding fuel channel end fittings 103 band 103 a.

As shown in FIG. 4C, the accept machine 108 is loaded with the fuelbundle surrogate 110 a, which includes a plurality of unirradiated Mo-99targets 122 (FIG. 3A), and an additional second half-string ofnon-fissile bundles 133 b. The non-fissile bundles 103 a and 103 b helpposition the fuel bundle surrogate 110 a in the desired location withinthe fuel channel 102 and, therefore, the reactor core 101. The desiredlocation within the fuel channel 102 is primarily dependent on thestrength of the flux field within the reactor core 101. As well, thenon-fissile bundles help maintain even flow of the primary fluid throughthe corresponding pressure tube 104. Next, the fuel bundle surrogate 110a is inserted into the pressure tube 104 in the “fuel against flow”manner, as shown in FIG. 4D, followed by the insertion of the secondhalf-string of non-fissile bundles 133 b in the “fuel against flow”manner, as shown in FIG. 4E.

After the fuel bundle surrogate 110 a is irradiated in the flux field ofthe reactor core 101 for an appropriate period of time, which isdetermined based on the type of radioisotope material being utilized(for example, seven days for Mo-99), as shown in FIG. 4F, the acceptmachine 108 is loaded with the next fuel bundle surrogate 110 b to beirradiated, as shown in FIG. 4G.

As shown in FIG. 4H, the second half-string of non-fissile bundles 133 bis unloaded in the “fuel with flow” manner, meaning the non-fissilebundles exit the pressure tube 104 in the same direction as the flow 107of the primary fluid, followed by the irradiated surrogate fuel bundle110 a.

As shown in FIG. 4I, the new unirradiated fuel bundle surrogate 110 b isinserted using the “fuel against flow” method, which is followed by thesecond half-string of non-fissile bundles 133 b being inserted into thepressure tube 104 of the fuel channel 102 in the “fuel against flow”manner, as shown in FIG. 4J. As previously noted, the non-fissilebundles 133 a and 133 b assist in positioning the fuel bundle surrogates110 a and 110 b in the desired locations and, as well, they help tomaintain proper flow distribution of the primary fluid through thepressure tubes 104. The method described above can be repeated on acontinuous basis during full-power operation of the reactor 100. Note,as described above, only one fuel bundle surrogate 110 is beingirradiated in the reactor core 101 at any given time. In alternatemethods, the number of fuel bundle surrogates 110 being activelyirradiated can vary as desired based on the desired schedule ofharvesting the irradiated radioisotope targets. Where it is desirable toyield fewer targets than contained in a fully-loaded fuel bundlesurrogate, filler materials may be utilized in place of a portion of theradioisotope targets.

Referring now to FIGS. 5A through 5I, another example method ofirradiating radioisotope targets in fuel bundle surrogates 110 is a“continuous use” method utilizing a dedicated fuel channel 104 of thereactor core 101. Referring now to FIG. 5A, the accept machine 108 ofthe reactor fuel loading equipment 106 is first loaded with a first setof four surrogate fuel bundles 110 a, which preferably include Mo-99targets. As well, the accept machine 108 is loaded with eightnon-fissile bundles 133. As in the previously discussed method, thenon-fissile bundles 133 assist in both positioning the surrogate fuelbundles in the desired location and maintaining proper flow distributionof the primary fluid through the pressure tubes 104 of the reactor core101. The accept machine 108 and charge machine 109 access the desiredfuel channel 102 by way of the corresponding fuel channel end fittings103 b and 103 a.

As shown in FIG. 5B, the first four fuel bundle surrogates 110 a areloaded in the “fuel against flow” manner, which is then followed byinserting the eight non-fissile bundles 133 into the pressure tube 104of the fuel channel 102 also using the “fuel against flow” method, asshown in FIG. 5C.

Referring to FIG. 5D, the charge machine 109 is then loaded with asecond set of four additional surrogate fuel bundles 110 b that alsoinclude a plurality of unirradiated Mo-99 targets. As shown in FIG. 5D,as the charge machine 109 inserts the second set of fuel bundlesurrogates 110 b into the pressure tube 104 in the “fuel with flow”manner, the accept machine 108 removes four of the non-fissile bundles133 in the “fuel with flow” manner (FIG. 5E).

As shown in FIG. 5F, the charge machine 109 is next loaded with a thirdset of four additional fuel bundle surrogates 110 c which are theninserted by the charge machine 109 into the pressure tube 104 in the“fuel with flow manner.” As the third set of four fuel bundle surrogates110 c are being inserted, the accept machine 108 removes the remainingnon-fissile bundles 133, as shown in FIG. 5G.

As shown in FIG. 5H, each bundle within the pressure tube 104 is now afuel bundle surrogate 110 a, 110 b, and 110 c including irradiated Mo-99targets. Also as shown in FIG. 5H, the charge machine 109 is next loadedwith a fourth set of four new fuel bundle surrogates 110 d that includeunirradiated Mo-99 targets. As shown in FIG. 5I, as the fourth set ofunirradiated fuel bundle surrogates 110 d is inserted in the pressuretube 104 in “fuel with flow” manner, the accept machine 108 removes thefirst set of four fuel bundle surrogates 110 a that have been irradiatedfor the desired time (that being approximately 7 days for Mo-99). Asshown in FIG. 5H and FIG. 5I, new fuel bundle surrogates may be loadedby the charge machine 109 when it becomes time to remove the fuel bundlesurrogates that have been irradiated for the desired amount of time,leading to the continuous nature of this method. Note, in alternatemethods, fewer or more than four fuel bundle surrogates may be includedin each set.

Another example method of irradiating radioisotope targets includes“reshuffling” an existing string of fissile fuel bundles within acorresponding fuel channel. A fuel string is reshuffled by removing afissile fuel bundle and replacing it with an unirradiated fuel bundlesurrogate. The position of the fuel bundle surrogate within the fuelchannel may be varied by removing a given number of fissile fuelbundles, inserting the fuel bundle surrogate, and then replacing all ofthe previously removed fissile fuel bundles with the exception of thelast one. As well, multiple fuel bundle surrogates may be inserted intothe fuel channel as long as a corresponding number of the removedfissile fuel bundles are not re-inserted into the fuel channel. Toremove the surrogate fuel bundles after an appropriate irradiationperiod, the fueling machine reshuffles the string at the output end(accept machine 108, FIG. 1C), and inserts an equal number of newfissile fuel bundles at the input end (charge machine 109, FIG. 1C). Tooptimize this method, the extraction of the irradiated fuel bundlesurrogates preferably occurs at the same time plant operators wouldnormally anticipate a routine fueling operation to provide new fissilefuel bundles to the fuel channel.

The radioisotope material irradiation methods discussed above offervarious advantages that may not be present in existing irradiationmethodologies, such as when irradiation ports of research reactors areutilized for target irradiation. Most notably, methods of irradiationthat include the utilization of fuel bundle surrogates, such as thosediscussed above, offer the ability to increase the amount of a givenradioisotope that may be produced over a given amount of time. Forexample, the full volume of a fuel bundle surrogate 110 (FIGS. 2A and2B) can be utilized to yield large quantities of irradiated materialrelative to existing methods of irradiating radioisotope targets (i.e.,using research reactors). As well, multiple fuel bundle surrogates 110can be irradiated simultaneously in a single fuel channel of a CANDUreactor, for example, up to 13 fuel bundle surrogates 110 at a time.Moreover, multiple fuel channels within a given CANDU reactor may beutilized simultaneously.

In addition to increasing yields of radioisotopes over existingirradiation methods, more than one radioisotope target material can beloaded into a single fuel bundle surrogate 110 for simultaneousirradiation. Similarly, different radioisotope target irradiationmaterials may be loaded into different fuel bundle surrogates 110 withina single string (fuel channel) of the reactor. Manipulation of thesefuel bundle surrogates 110 by the fueling machine 106 (FIG. 1C) allowsfor the different fuel bundles surrogates 110 to be irradiatedsimultaneously, yet removed from the corresponding fuel channelindependently of each other dependent upon the desired period ofirradiation for the given target material. As well, any fuel channelwithin the reactor core maybe utilized for the irradiation of fuelbundle surrogates 110 provided an adequate level of flux is present atthat fuel channel.

Referring now to FIGS. 6A through 6H, a system for moving irradiatedfuel bundle surrogates 110 from the reactor to a desiredreceiving/processing area within the reactor complex includes a shieldedcontainment flask 130 that is disposed on a movable trolley 131, and aconveyor 134 and extends between a receiving area 137 for the flask anda collection area 140 for the irradiated targets 122. As shown, theflask 130 is received on the trolley 131 so that the flask 130 can bemoved between a location in which it is accessible by the accept machine108 (FIG. 1C) and a location where the irradiated fuel bundle surrogates110 may be safely removed from the flask 130, such as the flaskreceiving area 137.

After one or more fuel bundle surrogates 110 have been irradiated andremoved from the reactor core 101 by the accept machine 108 (FIGS. 4Athrough 4J and 5A through 5I), the accept machine 108 is temporarilylatched onto the corresponding port (not shown) on the flask 130,wherein each port is in communication with a corresponding storagelocation within the flask 130. In a preferred embodiment, each port onthe flask 130 is constructed similarly to the previously discussed fuelchannel end fittings 103 a and 103 b (FIG. 4A), thereby allowing theaccept machine 108 to engage the flask 130 in the same manner it engagesa fuel channel of the reactor core. Each storage location within theflask 130 is configured to accept one or more irradiated fuel bundlesurrogates 110. Upon discharging one or more fuel bundle surrogates 110in to the interior of flask 130, the accept machine 108 is undocked fromthe flask 130 and returned to routine operations as needed.

Preferably, flask 130 includes adequate shielding such that theirradiated fuel bundle surrogates 110 can be moved about the reactorcomplex as needed to the desired location. Referring specifically toFIG. 6A, once trolley 131 has conveyed flask 130 to the desiredprocessing area, such as flask receiving area 137, an end door 132 offlask 130 is removed so that the irradiated fuel bundle surrogates 110may be removed. Note, because the fuel bundle surrogates 110 disposedwithin flask 130 have been irradiated, the end door 132 of the flask 130is only removed after necessary doors, interlocks, etc., have beensecured to prevent inadvertent exposure to plant personnel. Withcontainment established and the end door 132 removed, the surrogate fuelbundles 110 are removed from the flask 130 and transported on conveyor134 to the target collection area 140, as shown in FIGS. 6B and 6C.Preferably, each storage location within the flask 130 includes a ram(not shown) configured to urge the irradiated fuel bundle surrogates 110out of the flask 130. As best seen in FIG. 6D, each fuel bundlesurrogate 110 is received on a corresponding cradle 135 so that it issupported above the conveyor 134 and its motion along conveyor 134 isfacilitated.

As shown in FIGS. 6D and 6E, once the surrogate fuel bundle 110 isreceived at the target collection area 140, the end plates 120 areremoved from both ends of the fuel bundle surrogate 110. As shown, afuel bundle surrogate 110 includes end caps 114 that are secured to theend plates 120 so that each tube sheath 112 is open at both ends oncethe end plates 120 have been removed from the fuel bundle surrogate.Referring now to FIGS. 6F and 6G, with both end plates 120 removed, aramrod 136 is preferably used to push the irradiated Mo-99 targets outof the corresponding tube sheaths 112 and into the collection area 140.As best seen in FIG. 6F, the ramrod 136 includes a plurality of parallelarms 138, each arm 138 being positioned on the ramrod 136 so that itcorresponds to a single tube sheath 112 of the fuel bundle surrogate110. As such, the ramrod 136 is able to urge all of the targets 122 outof the fuel bundle surrogate 110 in a single pass. As shown, removal ofthe end plates 120 leaves the remainder of the fuel bundle surrogate 110intact so that it may be readily reused in subsequent target irradiationprocesses.

As discussed above with regard to FIGS. 2A, and 2B, one embodiment of afuel bundle surrogate 110 in accordance with the present disclosure isconstructed using the known design, materials, and construction methodsas existing fissile fuel bundles for CANDU reactors. Specifically, fuelbundle surrogate 110 includes tube sheaths 112, end caps 114, bearingpads 116, end plates 120, and spacer pads 118 that are constructed andassembled similarly to fissile fuel bundles, the primary differencebeing that fuel bundle surrogate 110 includes radioisotope targets 122(FIGS. 3A and 3B) rather than fissile fuel pellets. As such, fuel bundlesurrogate 110 affects the flow characteristics of the primary fluidwithin the corresponding fuel channel of the reactor core in the samemanner as the fissile fuel bundles. As well, as would be expected, theeffects of the primary flow and radiation on fuel bundle surrogate 110are similar to their effects on fissile fuel bundles 121.

Referring to FIGS. 7A and 7B, tube sheaths 200 of fissile fuel bundlesare designed to undergo sheath collapse in which a large portion of thesheath wall “collapses” onto the fuel pellets 202 that are disposedtherein. Whereas sheath collapse is desirable in fissile fuel bundlesbecause it reduces the gap 201 that exists between the inner surface oftube sheath 200 and the fuel pellets 202, thereby increasing thermalconductivity, it can be a disadvantage when removing irradiatedradioisotope targets from a tube sheath of the fuel bundle surrogate. Asshown in FIG. 7B, sheath collapse can lead to sag deformation regions113 that form at the axial ends of the tube sheath 200 adjacent to theend caps 204. The reduced diameter of the tube sheath 200 that occurs inthe vicinity of the sag deformation regions 113 can hamper removal ofthe irradiated radioisotope targets 122 from the corresponding tubesheath 200 in that the targets have a greater outer diameter than theinner diameter of the radiated tube sheath 200.

In order to avoid sheath collapse and the corresponding potentialissues, alternate embodiments of fuel bundle surrogates can includemodifications to the existing design of traditional fissile fuelbundles. For example, a modified fuel bundle surrogate may include atube sheath 112 that includes a thickened wall portion 112 a thatextends the length of the tube sheath 112, with the exception of thinnerannular end portions 117 disposed at each end of the tube sheath 112, asshown in FIG. 8A. The thickened wall portion 112 a provides additionalstructural stiffness for the tube sheath 112, thereby helping preventsheath collapse and its related issues. Annular end portions 117 arethinner than thickened wall portion 112 a and are configured to receiveend caps 114 by way of a welding operation. Whereas the thickened wallportion 112 a assists in preventing sheath collapse, the thinner annularend portions 117 facilitate the removal of end caps 114 by way of astandard operation, such as cutting, as shown in FIG. 9B. By preventingsheath collapse yet still allowing for the removal of the end caps 114by traditional cutting operations, the modified tube sheath 112facilitates the removal of radiated radioisotope targets 122 (FIG. 9B)by maintaining the structural integrity of the tube sheath.Additionally, prevention of sheath collapse assist in reducingcompression stresses on the contents (radioisotope targets 122) of thetube sheaths. In yet another alternate embodiment, as shown in FIG. 8B,the thickened wall portion 112 a extends for the entire length of thetube sheath 112.

Referring now to FIG. 10, embodiments of modified fuel bundlessurrogates may include features to facilitate visually distinguishingtube sheaths 112 that include radioisotope targets rather than fissilefuel pellets in order to prevent their unintended use in fissile fuelbundles. For example, tube sheaths 112 of modified fuel bundlesurrogates may include modified bearing pads 146 and/or additionalbearing pads 148 that are in excess of those found on typical fissilefuels bundles. As shown in FIG. 2B, standard bearing pads 116 oftraditional fissile fuel bundles are used to support the fuel bundlewithin the corresponding pressure tube of the reactor core. Additionaldistinguishing features may include 1-D or 2-D barcodes that are read bya corresponding barcode reader to obtain identification information ofthe tube sheath contents. Additionally, an appendage 142 can be providedon the outside surface of the modified tube sheath 112 that prevents themodified tube sheath from being handled by existing automated fissilefuel bundle assembly equipment. Preferably, one or more of theseidentification features are provided on modified tube sheaths 112 tohelp prevent the transport of fissile fuel bundles beyond authorizedareas and help ensure that only fuel bundle surrogates are transferredto the intended handling and/or processing areas.

Referring back to FIGS. 6D and 6E, another embodiment of a fuel bundlesurrogate in accordance with the present disclosure utilizes modularconstruction so that the fuel bundle surrogate 110 may be used inmultiple target irradiation processes, thereby reducing the amount ofirradiated scrap materials that must be handled upon completion of eachtarget irradiation process. As shown, modular fuel bundle surrogate 110includes the same components as the previously discussed embodiments offuel bundle surrogates, with the primary difference being that the endcaps 114 for the tube sheaths 112 are welded to the respective endplates 120 rather than the axial ends of the tube sheaths 112. As such,removal of the end plates 120 from the fuel bundle surrogate 110 alsoresults in the removal of the end caps 114, leaving the tube sheaths 112open at both ends. In short, end caps 114 create a fluid-tight seal withthe corresponding tube sheath 112 without having to be welded thereto,as discussed in greater detail below. Various configurations of endplates are possible as necessary for the modular fuel bundle surrogates110 to exhibit similar flow resistance to the primary flow as do thefissile fuel bundles. The end plates of modular fuel bundle surrogatesmay also be thickened in the axial direction to increase theirstructural integrity since they are reused for multiple radioisotopetarget irradiation processes.

Similarly to the previously discussed embodiments, the tube sheaths ofmodular fuel bundle surrogates may receive radioisotope targets 122(FIG. 3A) in which the radioisotope materials are encapsulated. As well,the tube sheaths may receive the radioisotope material directly in theform of powder, solid pellets, sheet stock, disc form, etc. Preferably,the tube sheaths 112 of modular fuel bundle surrogates include thickenedwalls 112 a, as shown in FIG. 12, as compared to the wall portions 212 aof the tube sheaths 212 used in traditional fissile fuel bundles (FIG.11). The thickened wall portions 112 a of modular tube sheaths 112provide additional structural integrity to increase the naturalfrequency of the tube sheath 112 beyond a level of concern for expectedprimary flow conditions, thereby avoiding vibrational concerns that mayarise regarding thin wall traditional fissile fuel bundles. Theincreased structural integrity and, therefore, reduced vibrations of themodular tube sheaths 112 can lead to reduced inter-element wear of thecomponents of a modular fuel bundle surrogate over traditional designs.

Referring now to FIGS. 13A through 13C, various systems for securing theend plates to the body of the modular fuel bundle surrogate 110 areshown. Referring specifically to FIG. 13A, a central tie-rod 160 that isattached to a first end plate 162 may be secured to a second end plate164 once the end caps 114 have been engaged with the corresponding endsof the tube sheaths 112. The tie-rod 160 can be secured to second endplate 164 by a removable fastener, a single use breakable restraint, aweld, etc. As shown if FIG. 13B, the embodiment shown in FIG. 13A can bemodified by providing slightly curved first and second end plates 162 aand 162 b, wherein the concave surfaces are disposed adjacent ends ofthe tube sheaths 112. As the first and second end plates 162 a and 164 aare drawn together by the central tie-rod 160 and begin to flatten out,they provide an additional spring force that enhances the axialintegrity of the modular fuel bundle surrogate 110. As shown in FIG.13C, an alternate embodiment of a modular fuel bundle surrogate 110 caninclude first and second end plates 172 and 174 that are secured toopposite ends of the plurality of tube sheaths 112 by spring lockassemblies 176. As shown, each end cap 114 is secured to thecorresponding end of a tube sheath 112 with an individual spring lockassembly 176.

Referring now to FIGS. 14A through 14B, in an alternate embodiment of amodular fuel bundle surrogate, the end plates may be secured to the tubesheath 112 utilizing a multiple re-weld design. As shown in FIG. 14A, aspot weld 151 is used to secure each end cap 114 to an end of acorresponding tube sheath 112, meaning the end plate is secured to thetube sheaths 112 by way of the end caps 114. As shown in FIGS. 14B and14C, after the target irradiation process is complete, the spot welds151 are machined away, thereby allowing the end plates to be removedfrom the tube sheaths 112 and the irradiated target material to beremoved. As show in FIG. 14D, prior to a subsequent target irradiationprocess, other unirradiated targets are disposed within the tube sheaths112 and the end caps are once again spot welded to the ends of thecorresponding tube sheaths 112.

Referring now to FIGS. 15A through 15C, various devices for maintainingthe rotational integrity of modular fuel bundle surrogates are shown. Asshown in FIG. 15A, the rotation of tube sheath 112 with regard to theend plate 120 can be prevented by use of a tab 178 disposed on the endcap 114 that engages a corresponding slot 180 defined in the tube sheath112 as the end cap 114 is slidably received therein. As shown in FIG.15B, the rotation of tube sheath 112 with respect to end plate 120 canbe prevented by a spring tab 182 that engages a corresponding hole 184defined by tube sheath 112 as end cap 114 is slidably received therein.A shown in FIG. 15C, rotation of tube sheath 112 with regard to endplate 120 can be prevented by the engagement of a pair of axiallyextending pins 190 and 193 that engage corresponding holes 191 and 192,respectively. As shown, pin 190 and hole 192 are disposed on the end oftube sheath 112, and pin 193 and hole 191 are disposed on end plate 120.In addition to the above systems, other methodologies such as temporarywelds, friction fit, non-circular cross-sections, interior key andkeyway arrangements, etc., between the tube sheaths 112 and end plates120 may be used to prevent relative rotation between the components.

As shown in FIGS. 16A through 16C, various arrangements may be utilizedto provide a seal for the tube sheaths of the modular fuel bundlesurrogate. As shown in FIG. 16A, a radiation resistant annular seal,such as an O-ring 300, may be received in a groove 302 that is formed inthe outer surface and end cap 114. Note, a sprung C-ring, or single usemetal crush gasket, may also be used in the embodiment shown in FIG.16A. As shown in FIG. 16B, a metallic seal can be formed between afrustoconical surface (not shown) formed on the inner wall of the tubesheath 112 and a mating frustoconical surface 304 formed on the end cap114. Sealing between the two surfaces can be enhanced by the use ofcompression supplied by one or more tie-rods (not shown) that extendbetween the end plates of the modular fuel bundle surrogate. As shown inFIG. 16C, an external swage ring 306 may be used to achieve aninterference fit with the corresponding end cap 114. The seal betweenthe tube sheath 112 and the end cap 114 may be enhanced by utilizing asoft coating (not shown) on the outer surface of the end cap 114.

Various embodiments of fuel bundle surrogates in accordance with thepresent disclosure may also include materials other than radioisotopetarget material within the individual tube sheaths, or fuel bundlesurrogate on the whole. For example, some materials that may be disposedwithin fuel bundle surrogates include, but are not limited to, isotopegenerating materials, materials for a radiation response research intesting, inert non-fissile filler, depleted uranium, composite fillersto adjust neutron density, as well as fissile material. In instanceswhere fissile material is included in a fuel bundle surrogate, visualand machine readable identification markers, such as those discussedwith regard to FIG. 10 above, are preferably utilized with each tubesheath 112 to prevent fissile materials from being improperlytransferred to lab or production facilities. As well, differentmaterials may be disposed along the length of each tube sheath. Forexample, differing materials may be disposed along the length of a tubesheath to affect the dynamic response of the tube sheath in the primaryflow.

Additionally, multiple research or isotope materials may be disposed inone element, or may be disposed in differing tube sheaths within thefuel bundle surrogate to take advantage of different flux due toself-shielding within the bundle. For example, neutron transparentmaterials may be disposed in outer rings of a fuel bundle surrogate,whereas denser materials are disposed toward the bundle center tooptimize distribution of flux within the fuel bundle surrogate or toreduce the impact of flux disturbance between adjacent fuel channels. Aswell differing lengths and densities of materials may be altered alongthe axial length of a tube sheath in order to maintain the desireddynamic response of the tube sheath within the primary flow. Forexample, lighter materials may be disposed at the ends of tube sheathswhere the impact to natural frequency is less, and heavy materials maybe disposed toward the middle of the tube sheaths. As well, shortersegments of materials may be utilized for higher flexibility, resultingin lower natural frequency. Similarly allowing for more clearancebetween materials and the tube sheath wall allows for higher flexibilityand, therefore, lower natural frequency.

While one or more preferred embodiments of the invention are describedabove, it should be appreciated by those skilled in the art that variousmodifications and variations can be made in the present inventionwithout department from the scope and spirit thereof. It is intendedthat the present invention cover such modifications and variations ascome within the scope and spirit of the appended claims and theirequivalents.

What is claimed is:
 1. A method of irradiating a target material in a heavy water reactor for the production of an isotope, comprising the steps of: providing a first target comprised of a first target material suitable for producing the isotope by way of a neutron capture event; placing the first target in a primary fluid side of the heavy water reactor; and irradiating the target.
 2. The method of claim 1, wherein the heavy water reactor is a CANDU reactor.
 3. The method of claim 1, wherein the first target is disposed within a surrogate fuel bundle.
 4. The method of claim 3, further comprising the step of inserting the surrogate fuel bundle into a pressure tube of the reactor.
 5. The method of claim 4, further comprising the step of removing the surrogate fuel bundle after a predetermined residency time in a flux field of the reactor, whereby the first target is irradiated.
 6. The method of claim 5, further comprising transferring the irradiated first target to a processing facility.
 7. The method of claim 1, wherein the step of providing a target further comprises: providing a fuel bundle surrogate including a plurality of tube sheaths that extends between a pair of opposed end plates; and placing the target into a first one of the tube sheaths.
 8. The method of claim 7, further comprising providing a non-fissile material, wherein the non-fissile material is disposed in one of the first one or a second one of the tube sheaths.
 9. The method of claim 1, further comprising the steps of: providing a second target comprised of a second material suitable for producing a radioisotope by way of a neutron capture event; and placing the second target into a second sheath tube of the plurality of sheath tubes of the surrogate fuel bundle.
 10. The method of claim 1, wherein the step of providing a first target further comprises providing an outer capsule defining an interior volume, wherein the first target material is disposed in inner volume.
 11. The method of claim 10, wherein the outer capsule is comprised of the first target material.
 12. A fuel bundle surrogate for the irradiation of a target material in a heavy water reactor for the production of an isotope, the fuel bundle surrogate comprising: a plurality of tube sheaths, each tube sheath being parallel to a longitudinal center axis of the fuel bundle surrogate; a plurality of end caps, each end cap being disposed on a corresponding end of a tube sheath; a pair of end plates, wherein the end plates are disposed at opposing ends of the plurality of tube sheaths; and a first target comprised of a first target material suitable for producing the isotope by way of a neutron capture event, wherein the first target is disposed in a first tube sheath of the surrogate fuel bundle.
 13. The fuel bundle surrogate of claim 12, wherein the first target further comprises an outer capsule defining an interior volume, wherein the outer capsule is comprised of the first target material and the first target material is disposed in the interior volume of the outer capsule.
 14. The fuel bundle surrogate of claim 13, wherein the first target material disposed in the interior volume of the outer capsule is in the form of powder, disks, flasks or pellets.
 15. The fuel bundle surrogate of claim 12, further comprising a second target comprised of a second target material, wherein the second target is disposed in a second tube sheath of the fuel bundle surrogate.
 16. The fuel bundle surrogate of claim 12, further comprising an identification indicia on an outer surface of the first tube sheath, wherein the identification indicia is one of a bar code, an appendage or a bearing pad.
 17. The fuel bundle surrogate of claim 12, wherein each end cap is rigidly affixed to the corresponding end of the corresponding tube sheath by a weld.
 18. The fuel bundle surrogate of claim 12, wherein each end cap is secured to the corresponding end of the corresponding tube sheath by force exerted therein by a corresponding end plate. 